The Role of Microorganisms in the Pathogenesis of Pulpal and Periapical Disease It has been demonstrated that inflammation in the periodontal ligament may be induced by specific combinations of bacteria in the root canal and the presence of Porphyromonas endodontalis in such combinations is essential. The studies currently being carried out are based on previous observations that some indigenous oral bacteria are capable of inducing apical periodontitis or apical abscesses and that certain combinations of bacteria are more potent in inducing symptoms and pathosis than single strains. The specific aims of this study are: 1) to determine if specific bacteria become dominant in mixed cultures, 2) to determine if growth stimulatory and/or inhibitory factors are produced by specific bacteria in mixed cultures, 3) to determine if Endodontic pathogens differentially express cell surface proteins/capsules and/or elaborate potential virulence factors upon growth in the presence of sterilized culture filtrates or purified "factors" from other microorganisms. The research design and methods include examination of the growth kinetics of the organisms in batch and mixed batch cultures. Information from these studies will be utilized in examining microbial interactions associated with pulpal and periradicular pathosis in mixed continuous cultures. One possible ecological determinant that has been examined is the effect of atmospheric hydrogen concentration on the growth rate and yield of Porphyromonas endodontalis in batch culture. P. endodontalis ATCC 35406 exhibits a substantial increase in both growth rate and yield when grown in an anaerobic atmosphere with greater than 10% Hydrogen. KEY WORDS: Ecology, Kinetics, Pathosis